The invention is directed to a lighting fixture for use in ceilings, an attachment to ceilings, and suspended from ceilings having a box-shaped housing. A grid of lamellae having at least an approximately double-parabolic cross-section connected to one another lattice-like is inserted into a light exit opening at the fixture which, given the lowest possible masking losses of the lamps arranged above the grid in the housing, produces the shielding desired in accordance with the respective demands of the workplace or in the room.
Lighting fixtures of this type are known, for example, from the reference Siemens-Elektrodienst, volume 22, number 3, April 1980, pages 4 and 5. Interior lighting fixtures for picture screen work stations must meet specific shielding conditions according to a BAP condition. The BAP condition realized in these lighting fixtures means that their luminance in the 90.degree. angular range between vertical and horizontal is subdivided into two regions, namely into an emission region having an emission angle gamma equal to 50.degree. and into a dark region having a shielding angle beta equal to 40.degree.. The luminance must remain, below 200 cd/m.sup.2 in the dark on the quality of illumination needed for a room without regard to BAP, the division of the 90.degree. angular range into an emission region and a dark region can deviate from the angular values prescribed in the BAP condition.
Given long field lamps, the dark region in the direction perpendicular to the axis of the fluorescent tube is achieved by an appropriately shaped, channel-like reflector having a parabolic cross-section. The dark region in the direction of the axis of the fluorescent tube is achieved by a grid of lamellae having a double-parabolic cross-section inserted into the light exit opening, these lamellae being arranged parallel to one another at mutually identical distances and perpendicular to the axis of the fluorescent tube. Taking the height and the cross-section of the lamellae into consideration, the mutual spacing of the lamellae is selected such that the desired shielding is achieved given the least possible masking of the light of the fluorescent tube.
For achieving an omnidirectional characteristic, such long field lamps meeting a specific shielding condition can in fact be utilized in such fashion that a plurality of long field lamps are annularly arranged in the fashion of an optical conical pattern. However, a ring arrangement is only possible when an adequately large ceiling area is available. Further, lighting fixtures having a rotational-symmetrical light exit opening are known, for example, from the references German utility model 19 63 808 and DE 29 26 202 Al, wherein the light exit opening is provided with a rotational-symmetrical lattice-shaped grid. These grids, however, have only a decorative effect and do not meet any specific shielding conditions.